Production of porous metal plates



Sept. 14, 1954 w |-||GNETT 2,689,178

PRODUCTION OF POROUS METAL PLATES Filed March 24, 1949 X i i I I G- N k5 k L Afton/76y Patented Sept. 14, 1954 UNITED STATES ATENT OFFICE PRODUCTION OF POROUS METAL PLATES Application March 24, 1949, Serial No. 83,123

Claims priority, application Great Britain March 25, 1948 1 Claim.

This invention relates to the production of porous metal plates.

Porous metal plates of high porosity may be produced by sintering metal powders in a reducing atmosphere. In order to obtain high porosity, that is to say, to make the voids constitute at least 55% and even from 80 to 90% of the volume, it is usual to form the plates with the aid of a spacing agent which volatilises during the sintering operation. This procedure however has the disadvantage that the size and shape of the pores in the sintered metal are affected by the size and shape of the particles of spacing agent, which are difficult to control. Further, if the gases that are evolved are allowed to escape freely during the sintering the plate may be distorted and if the gas is only allowed to escape through restricted openings the powder may be displaced before it sinters. The need for a spacing agent can be obviated by employing metal powders which, because a mass of the powder contains a large number of voids, are of low bulk density. These powders are typified by nickel powder prepared by decomposing nickel carbonyl. The nature of the powder and its bulk density depend upon the conditions under which the carbonyl is recomposed and the powders can be of two difierent kinds, which may respectively be called A-nickel and B-nickel. The conditions under which B-nickel powders are produced by the thermal decomposition of nickel carbonyl are indicated in the prior art by the article entitled German Iron and Nickel Powders by T. P. Colclough published in The Iron Age of February 21, 1946, pages 48 and 49. A-nickel powder consists essentially of particles which may vary in size from 2 to 15 microns and which form compacts in which the particles are mechanically but very loosely held together. The bulk density of A-nickel powder varies from 0.6 to 4.5 grams per cubic centimetre. B-nickel powder consists essentially of smaller particles than A-nickel, but these particles are interlocked into aggregates which are not only stronger than the loose compacts of A-nickel but also are themselves interlocked. Accordingly the bulk density of B-nickel powder is lower and varies from about 0.3 to about 1.2 grams per cubic centimetre. The lower the bulk density, the higher the porosity of the final sintered product, but at the same time the higher is the contraction during sintering.

Because of the different natures of A-nickel and B-nickel the porosities of plates made from them difier. High porosities can be obtained with B-nickel despite the contraction during sintering.

2 A-nickel, on the other hand, tends to crack during sintering and if it is first compressed to an extent sufficient to eliminate cracking the porosity is reduced to a comparatively low figure, which may be no more than 50%.

The contraction which takes place during sintering leads to distortion, but undistorted plates of uniform size, thickness and shape can be produced if the direction of the contraction is controlled. Control may be efiected by confining the contraction substantially to one direction in such a way that the mass of powder is reduced in thickness during sintering, but not in length or breadth. For this purpose a wiremesh grid may be incorporated, but a grid cannot be introduced into a thin plate without great difficulty. Even if a thin plate is made with a grid in it, the grid causes loss of porosity and increases the weight.

An object of this invention is to produce porous metal plates of uniform dimensions and containing no internal wire grid or like device.

Another object of this invention is to produce highly porous metal plates by sintering in an improved manner.

A further object of the invention is to confine the contraction undergone by metal powders during sintering.

Broadly stated, in this invention a plate is built up by successively sintering a number of layers of metal powder, each being united to the preceding one by the sintering.

In the preferred way of carrying out the invention metal powder is delivered from a charging device onto a flat support to form a very thin layer, the layer is subjected to a sintering treatment, another very thin layer of metal powder is delivered onto the first, the powders on the support are again subjected to a sintering treatment, and the process is repeated one or more times. The support must, of course, be heat-resistant and in order to prevent contraction of the first layer during sintering the support preferably has a rough or corrugated surface, as described in the application of Mervyn Arthur Comley, Serial No. 83,164 filed March 24, 1949, and its divisional application, Serial No. 306,588 filed August 27, 1952. The foregoing U. S. application Serial No. 83,164

is now abandoned. Preferably it is made of an unglaz'ed ceramic material. With the use of such a support the first layer can be sintered into a coherent mass without contraction in a direction parallel to the surface of the support. The further thin layers of powder can then be sintered in position without causing distortion or unde- 3 sired contraction. Since contraction takes place mainly during the initial stages of sintering only a small loss of porosity need result from the fact that the layers first deposited have finally been subjected to several sintering treatments. Preferably the layers or all the layers but the last are sintered together at a relatively low temperature and then all the layers are subjected as a whole to further sintering at a higher temperature. This procdure presents the advantage that the mechanical strength of the product is increased by the additional sintering whichtakes place at the higher temperature but there is very little contraction of the whole plate during this sintering because nearly all the contraction has already taken place.

If high porosity is to be obtained the powder used should be of the B-nickel type.

One or more of the layers may be made from powder different from that used for one or more other layers so as to .form a plate'in which the size of the pores is different at different points in its thickness; plates somadeare useful for a variety of purposes.

In carrying out the invention the supports on which the plates are formed may travel, preferably reciprocating between a charging device and a furnace and passing through a cooler on the way. For this'purposea reciprocatory carrier may be combined with a charging device and a furnace.

Apparatus with the aid of which the process may be carried out are shown diagrammatically in the accompanying drawings in which Figure 1 shows an apparatus in which the layers are sintered at a relatively low temperature andFigure 2 an apparatus in which the sintering is completed at a higher temperature.

The apparatus shown in Figure 1 comprises a rack I which canbe reciprocated by a pinion 2. The rack is made of a nickel-chromium alloy and it carries a flat plate 3 which formsthe support on which the layers are sintered. Initially. this support 3 lies beneath ahopper 4, the base'of which is constituted by a vibrating screerr5. This screen isvibrated for long enough to cause powder to flow through it from the hopper andform a very thin layer on the support 3. The rack-is then moved to carry thesupport beneath: a knife blade 8, which is adjustable in heightandis reciprocated transversely to level the powder layer, and to remove excess when the last layer is applied. Upon continued movement .of the rack the support .3 is'carried through a water-cooled chamber 1 into a high-frequency induction furnace 3 in which an atmosphereof hydrogen is maintained. Current is induced in the powder layer to bring the temperature of thelayer to about 800 C., the supportand layer being kept in the furnace for a period of 5 minutes. This brings about light sintering of the layer. .The current supply to the furnace is then cut off and the rack l is moved backwards, so that the hot layer and support are carried through the chamber l and cooled in it. The support is then carried back to its starting position beneath the hopper 4 and another layer of powder is formed. When all the required layers have been formed and sintered the rack. may be moved tov bring the support and plate to the position marked 9 and the plate and support arethen removed. They are put on an endless belt I0 (Figure 2) made of 4 a nickel-chromium alloy and carried by it into a high-frequency induction furnace I I. Here the plate is brought to a temperature of 900 0., being kept in the furnace for 10 minutes, so that further sintering takes place. The belt I0 then carries the plate and support through a watercooled chamber 12 and on leaving this the plate is-removed as a finished article.

Other forms of apparatus may be used. For instance a series of charging devices and resistance-type sintering furnaces may be employed with either a continuous chain of moulds or a continuous nickel-chromium strip upon which the metal powder is deposited. The nickelchromium strip should be prepared for each complete .cycle of operation by treatment with a slurry of magnesia or alumina. This form of apparatus is suitable when different powders are to be used in making a plate. Again, other forms of charging device may be used.

As an example a plate in which the porosity is 85% and the thickness 2 mm. may be made from B-nickel having a bulk density of 0.7 grams per cubic centimetre. It is found that a first layer made of B-nickel tends by its nature to curl up at the edges if it is more than 0.5 mm. thick, so the first layer should be thinner than this. The total number of layers to produce a plate 2 mm. thick may be six.

Although reference has been made to nickel powder, iron or other powders of similar physical shape and density can be used.

The invention is particularly useful in the manufacture of plates for use in nickel alkaline accumulators.

I claim:

A method for preventing distortion in the production of a sintered highly porous nickel plate more than 0.5 millimeter thick having at least porosity, which comprises producing a first thin, sintered nickel layer not more than 0.5 millimeterdeep upon a support having a rough upper surface by sintering thereon a layer of loose B-nickel powder having its upper surface exposed and free from downward pressure; and after said first sintered layer is produced, successively sintering additional thin layers of loose B-nickel powder, layer on layer, onto said first sintered layer, whereby distortion is prevented in the production of an integral highly porous nickel plate more than 0.5 millimeter thick having at least 80% porosity.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,930,287 Short et al. Oct. 10, 1933 2,198,042 Schlecht Apr. 23, 1940 2,198,253 Koehring Apr. 23, 1940 2,198,702 Koehring Apr. 30, 1940 2,214,104 Hildabolt Sept. 10, 1940 2,297,817 Truxell Jr. Oct. 6, 1942 2,300,048 Koehring Oct. 27, 1942 2,332,746 Olt Oct. 26, 1943 2,368,458 Engle Jan. 30, 1945 2,544,112 Schneider Mar. 6, 1951 2,554,343 Pall May 22, 1951 OTHER REFERENCES Jones: Powder Metallurgy, published by Edward Arnold & 00., London, 1937, pages 21, 26 and 27. 

